1. Field of the Invention
The present invention relates generally to an apparatus and methods for foundation underpinning. More particularly, the invention relates to an apparatus and method for employing a foundation piling system to support and level an existing building foundation.
2. Background of the Invention
Several methods and systems have been developed and used for lifting, leveling and stabilizing above-ground structures such as buildings, slabs, walls, columns, etc. One conventional technique employs a stack, or pile, of pre-cast concrete pile segments that is positioned underneath, and supports, the structure to be stabilized and leveled. Typically, a hole is dug underneath the structure to a depth slightly greater than the length of a pile segment. Multiple pile segments are then driven into the ground one on top of the other until a particular depth is reached, thereby forming a vertical stack, or pile, of the pile segments. The pile segments are driven into the ground until a rock strata is encountered, or until the resulting pile is believed to be sufficiently deep to adequately support the structure. In situations where a rock strata cannot be reached, the pile segments are driven to a depth great enough to cause sufficient friction between the earth and the outer surfaces of the pile to prevent substantial vertical movement of the pilings. A jack is next positioned on the upper surface of the pile, between the uppermost pile segment and the structure. Finally, the structure is raised to the desired height.
There are several disadvantages to this conventional technique. Other than being stacked one on top of another, the concrete segments are typically not connected. Thus, the individual pile segments may become misaligned during installation. In some cases, substantial misalignment can negatively impact the stability and strength of the entire pile. In addition, determining the installed pile depth typically requires monitoring the quality and number of pre-cast concrete segments used to form the pile. Further, in most cases, the completed pile is not reinforced. Over time, the cyclical shrinking and swelling of the soil surrounding the piles can cause shifting of individual pile segments, potentially resulting in misalignment and weakening of the pile. Still further, the individual pre-cast pile segments are often cylindrical in shape. As each pile segment is driven into the ground, the entire outer radial surface of each pile engages the surrounding earth, resulting in relatively large frictional forces which can inhibit continued advancement of the individual pile into the ground.
Another conventional method utilizes a flexible cable to lock the individual pile segments together as a unit, thereby reducing misalignment of the concrete segments. A typical example of this methodology is found in U.S. Pat. No. 5,288,175. A starter concrete pile segment with a high strength steel cable anchored to and extending from the center of the starter segment is first driven into the soil beneath the foundation using a hydraulic jack. Multiple concrete segments, each having a central throughbore, are then sequentially threaded onto the cable and driven into position, each one on top of the other to form the complete pile that is used to support and level the structure. The cable is intended to promote vertical alignment of the pile segments. It also permits pile penetration depth to be determined, either by reading a strand marker or calculating it by measuring the length of cable used to lock the pile together.
In an effort to drive the individual pile segments deeper to achieve a more stable pile, and to reduce the time required to drive the pile segments, some conventional underpinning methods jet or spray a fluid into the soil beneath the lowermost pile segment. A typical example of this installation method is found in U.S. Pat. No. 5,399,055. Similar to other conventional methods, the individual pre-cast concrete segments are pressed or driven vertically into the soil using a hydraulic jack. When the concrete segments cannot be driven further, fluid is injected downward through holes formed in the concrete segments. The fluid moistens and loosens the soil beneath the pile, allowing the pile to be driven deeper and to be driven deeper more easily than would have otherwise been possible. After discontinuing the fluid jetting, additional concrete segments are positioned on the pile and driven downward using the hydraulic jack. Fluid is again injected through the concrete segments once the pile cannot be driven further downward. After the fluid jetting is discontinued, additional concrete segments positioned on the pile and driven downward, and so on. This process is repeated until the desired pile depth is reached. To promote alignment of the concrete segments, a reinforcing rod may be inserted into the holes formed in the concrete segments. Finally, in some cases, grout is injected into the annulus formed between the outer surface of the reinforcing rod and the inner surface of the holes through the concrete pile segments in an effort to solidify the pile as a unitary structure.
These relatively advanced methods of fluid jetting, however, are not without disadvantages. In particular, alignment of the pile segments is not always assured. When the pile segments are driven in, they are in no way connected, such as by a cable or reinforcing rod. As a result of such misalignment, jetting fluid may not reach the base of the pile. Therefore, the soil beneath the pile may not be moistened by the jetting fluid, preventing the pile from being driven as deep as desired. Also as a result of pile segment misalignment, it may not be possible to later insert the reinforcing rod through the entire depth of the pile. Nor may it be possible to inject grout the full length of the pile. Thus, the pile may be misaligned as well as not reinforced over portions of its length.
Accordingly, there remains a need in the art for a foundation underpinning apparatus and methods that offer the potential to maintain the alignment of the individual pre-case pile segments forming a pile both during and after installation. Such a foundation underpinning would be particularly well received if it could be installed deeper and more efficiently than known installation methods permit.